It is known to measure the speed of vehicles by means of Doppler radar. When an emitted radar wave is reflected at a moved surface, then, due to the Doppler effect, the wavelength and thus the frequency of the reflected wave is varied relative to the wavelength and the frequency of the emitted wave. This frequency variation is proportional to the speed of the moved surface toward the radar transmitter or away from the radar transmitter. This phenomenon is used in traffic monitoring devices in order to detect exceeding of a predetermined upper speed limit. In order to obtain evidential documents for an exceeding of the predetermined upper speed limit, it is furthermore known to photograph the vehicle, when such an exceeding is detected. Particularly also the license plate of the vehicle is photographed.
With such traffic monitoring devices, a radar transmitter emits a divergent radar beam at a small angle to the longitudinal direction of the road. A measuring section is defined on the road by this divergent radar beam, a vehicle driving on the road being detected by the radar beam within this measuring section. A measuring value of the speed of the vehicle is formed from the radar signals reflected at the vehicle. A photographic camera covers an aperture angle, within which the radar beam and the measuring section are located. The camera is automatically released when said measuring value of the speed of the vehicle exceeds a predetermined threshold value. Then the display of the measuring value is reflected into the picture or recorded in some other way (CH-PS 480,697, U.S. Pat. Ser. No. 3,522,611). It is also known to arrange a row or a matrix of luminous diodes in the area of the film rim, these diodes being controlled during the film feeding by a memory memorizing the measuring value of the speed, such that the measuring value is digitally recorded on the film rim (U.S. Pat. Ser. No. 4,053,909, DE-Pat. Ser No. 3,034,161).
The aperture angle of the camera must not be too large. The camera shall namely particularly record the license plate of the vehicle such that it can be clearly identified. Thus, an objective of long focal length having a correspondingly narrow field of view has to be used. This results in some problems:
Generally the photographed vehicle is not alone on the road. Other vehicles are present, which generally move at different speeds. Passing actions take place. These other vehicles can considerably disturb the radar measurement. They can possibly feign an exceeding of the upper speed limit, which is not present in reality or in any case not done by the driver of the photographed vehicle. The evidential value of radar measurement and photographic picture can thereby be reduced very much. This is particularly the case when the radar measurement just makes an "instant picture" and thus measures the speed just within a relative short period of time.
In order to at least recognize these errors, it would be necessary to photograph a larger field of view. Then it could be recognized, whether there were other, for example passing vehicles in the area of the photographed vehicle presumed to be measured, which other vehicles could have falsified the measurement. This requirement of recording a larger field of view stands in contradiction to the requirement of a clear picture of the license plate of the vehicle. The simultaneous recording by two cameras, one of which operates with an objective of short focal length and the other one of which operates with an objective of long focal length, requires considerable technical expenditure and doubles the film consumption.
Another way of recognizing erroneous measurements effected by other vehicles consists in that the speed measured by Doppler radar is followed up over a period of time as long as possible. The measurement should thus, if possible, be effected throughout the total measuring section, that is as long as the vehicle is hit by the radar beam. From the signal waveform over a time as long as possible, possible disturbing influences can then be recognized more easily.
Practically each Doppler period supplies a measuring value of the speed of the detected vehicle. Thus, series of individual measuring values are obtained. These measuring values can vary considerably due to different influences. A method is known for forming a measuring value to be recorded from these different varying measuring values and for elimination of doubtful measuring values. In this method the measuring values are classified according to speed classes. Thus, it is examined, how often a measuring value lies, for example, between 40 km/h and 50 km/h, how often between 50 km/h and 60 km/h etc. This results in a "histogram", that means a representation of the number of the speed measurements as a function of the measuring result. That part of the histogram is examined, which is located above a certain frequency level. With an accurate measurement one measuring value should dominate, that means that many measurements should result in this measuring value. The variation should be small. In this case the histogram would be a high narrow peak. Very wide histograms or histograms having several peaks and gaps therebetween indicate disturbances. In the first case the value of the speed, at which the high narrow peak is located, is taken as measuring value to be indicated. In the latter cases the measurement is rejected. In the known method the measurement is carried out over the total measuring section.
In the known method the traffic moving away is measured. This does not present any problems, after the measurement has been effected and evaluated, to make the photographic picture, when the vehicle with its rear leaves the radar beam, if the measurement indicates unambiguously exceeding of the upper speed limit. Then the rear of the vehicle is located at the limit of the radar beam and the measuring section, this limit being located within the field of view.
However, it is often desirable to monitor the approaching traffic, that means the vehicles travelling towards the radar beam in the direction toward the traffic monitoring device. This necessity can result from practical conditions. The monitoring of the approaching traffic also offers the advantage, that the vehicle can be photographed frontally. Thereby, using suitable exposure techniques, not just the license plate of the vehicle can be identified but also the driver.
If, however, it is desired to monitor the approaching traffic in the described way, then problems arise with the photographic recording: When the vehicle has completely passed the measuring section, such that its rear is located at the limit of the measuring section this time located on the side of the traffic monitoring device, then, due to the narrow field of view of the camera, the front end of the vehicle with the license plate of the vehicle generally is already located outside the field of view. Thus, the license plate of the vehicle could not be detected any more.
Therefore it is known, in the measurement in approaching traffic, to effect the measurement of the measuring value of the speed just in a narrow part of the measuring section actually available and to release the camera due to this measurement, when the measuring value exceeds the predetermined threshold value. Then the measuring value obtained with this measurement also is recorded. This results in the disadvantage already mentioned above, that practically just an instant value of the speed measured with the Doppler radar is obtained, which does not permit any conclusions about disturbing influences and the reliability of which can therefore be doubtful.
Therefore, a method for photographic traffic monitoring is known (EP-A- 0,188,694), wherein the releasing of the camera in fact is controlled by such a relative short-time speed measurement, such that the front end of the vehicle still is detected in the field of view of the camera, and with which also the measuring value obtained with this short-time speed measurement is recorded, wherein, however, subsequently a "verification" of this measuring value is effected. Over the rest of the measuring section it is examined whether the arriving speed measuring values are within a predetermined band width about the first measuring value to be recorded. If this is not the case, this measuring value is not recorded, that means it is not reflected into the picture already made.
Also with this known method the measuring value of the speed to be recorded is obtained just on a part of the available measuring section. Therefore the measuring value is not completely reliable, as explained above. However, the "verification" during the remaining part of the measuring section uses this not quite reliable measuring value as a reference.